1. Field of the Invention
This invention relates to a vacuum pump having a function of increasing pressure for vacuum suction and pressure transport by rotating a pair of screw rotors.
2. Description of the Related Art
In technology of air-transporting powder and solid contents (such as cutting, wet refuse, dust, ash, coal, sludge, sand, cement and wheat flour) by a vacuum pump, gas pressure trends to be increased to 2-3.5 Kg/cm2G in accordance with reduction of pipe diameters and higher-density transport for long-distance transport or volume transport to reduce an initial cost thereof.
Such range of the gas pressure is out of the range of normal blower pressure (lower than the above pressure range) and the pressure range of a compressor (higher than the above pressure range, 7-8 Kg/cm2G so that when a blower is used for air transport, the blower may be a multi-stage type and when a compressor is used for air transport, the gas pressure may be reduced so as to correspond to the range of gas pressure.
For air transport, a method by vacuum suction and pressure transport is generally used. In the case, both of a vacuum pump and a compressor are required. For example, powder is sucked into a separator tank by the vacuum pump and transported with compressed air by a blower (pressure increase less than 1 Kg/cm2G) or by a compressor (pressure increase more than 1 Kg/cm2G) while the powder in the tank is dropped at constant rate of volume in a pipe by a rotary valve.
When a usual screw-type vacuum pump is used as a compressor, a screw rotor is loaded with a thrust force (axial force) Fa/4*(Da2−Db2)*Pd by receiving discharge pressure Pd. The thrust force is added on a bearing at a fixed side of the screw rotor so that life of the bearing may be reduced extremely.
Herein, Da is an outer diameter of a screw, Db is a root diameter of the screw and Pd is discharge pressure. For example, a vacuum pump which has a life Lh=30,000 hours only for a vacuum pump may have a extremely shorten life Lh=few thousands hours when the vacuum pump is used for a compressor with discharge pressure of 3 Kg/cm2G.
Therefore, increasing a shaft diameter of the rotor for a larger bearing, the root diameter of the screw is increased so that discharge air volume at one rotation of the screw rotor is reduced. Increasing rotating speed of the screw rotor for compensating the reduction, vibration and noise will be increased and lubricity might be increased. Alternatively, enlarging the outer diameter of the screw rotor for increasing the discharge volume, size of the pump might be increased.
In order to overcome the above drawback, the object of this invention is to provide a vacuum pump having a function of increasing pressure which can have a longer life of the bearing when the pump is used as a compressor for pressure of 2-3.5 Kg/cm2G and also can be used as a vacuum pump by closing an inlet thereof.